The Moderates' contacts with People's Party predecessors had started in 1998. The two parties had a joint list in 1999 parliamentary election and formed a governing coalition with Pro Patria Union and Reform Party. In 2003, the Moderate People's Party joined the Party of European Socialists. After disappointing election results in 2003, the party renamed itself the Social Democratic Party (SDE) on 7 February 2004.

After last elections to the local government councils on 16 October 2005, the party in most major cities is in opposition, but are a part of the governing coalition in Rakvere and Tapa. The party improved its position in most areas. In Tallinn, it formed a joint list with the agrarian People's Union, which got 6 seats out of 63 seats with 11.1% share of votes. Comparing to 2003. elections to the local government council in Tallinn SDE and People's Union gained seats. In the 2003 election, SDE got a 4.9% share of votes and People's Union 3.4% share of votes, which were both below a 5% election threshold. In Estonia, SDE local lists won 6.43% share of votes. In 2003 they got only 4.39% share of votes nationally. SDE is today represented on 65 local councils out of 206 and is governing in 20 local councils. After the 2003 election the party was represented in 104 local councils out of 247.

SDE's aim in the 2007 Estonian parliamentary election was to win at least 17 seats out of 101. Independent member of current RiigikoguLiina Tõnisson ran as a candidate in their list. All SDE's MEPs and their current MPs were candidates in the 2007 election. The party got 58,354 votes (10.6% of the total), a gain of +3.6%; it won 10 seats in the new Riigikogu, a gain of four.

In the 2015 parliamentary election on 1 March 2015, the SDE received 15.2% of the vote and 15 seats in the Riigikogu.[11] After the coalition formation with Reform and IRL, MP Jevgeni Ossinovski announced that he would challenge Sven Mikser in the party congress on 30 May 2015.[12] Mikser however stepped down before the election at the congress and Ossinovski was chosen as the new party leader.[4] On 7 November 2016, the Social Democratic Party and IRL announced that they were asking Prime Minister Taavi Rõivas to resign and were planning on negotiating a new majority government.[13] In the following vote of confidence on 9 November, the majority of Riigikogu voted in favor of removing the prime minister’s government.[14]

On 23 November 2016, a new coalition government consisting of the Estonian Centre Party, SDE and IRL was sworn in.[15]

1992–1994, 1994–1995, 1999–2002, 2007–2009 they participated in three centre-right governments with today's Fatherland Union and Reform Party. Social democrats were in government in 1992–1994 Mart Laar's first government, in 1994–1995 Andres Tarand's government (often called to Christmas Peace government (jõulurahu valitsus)) and in 1999–2002 Mart Laar's second government (often called Triple Alliance (kolmikliit)) and from 2007 to 2009, when the Social Democrat ministers were expelled from the government.